1. Field of the Invention
This invention relates to a photoconductive member which is sensitive to electromagnetic waves such as light (in the broad sense of the term, this includes ultra-violet rays, visible light rays, infrared rays, X-rays, .gamma.-rays, and so forth).
2. Description of the Prior Arts
To constitute a photoconductive layer for an electrophotographic image forming member, image original reading device, and so on, used in a television image pick-up tube, solid-state image pick-up device, etc., or in the field of the image forming technique in general, a photoconductive member is required to have various characteristics such as high sensitivity, high S/N ratio (photo-current (I.sub.p)/dark current (I.sub.d)), a spectral characteristic specific to an electromagnetic wave, with which it is irradiated, harmlessness to human body when it is used, and, in the case of the image pick-up device, capability of readily eliminating a residual image within a set period of time, and so forth. Particularly, in the case of the electrophotographic image forming member to be incorporated in an electrophotographic apparatus used as an office machine, this harmlessness to the human body on its use is of utmost importance.
From such standpoint, amorphous silicon (hereinafter abbreviated as "a-Si") has drawn attention of all concerned in the field of the photoconductive material. At the initial stage of its development, this a-Si from exhibited varying electrical and optical characteristics, because its structure is governed by method and conditions for its manufacture (vide, for example, Journal of Electrochemical Society, Vol. 116, No. 1, pp 77-81, January 1969), hence a serious problem in its reproducibility. In more detail, at the outset of its development, the a-Si film formed by the vacuum evaporation method and the sputtering method had a great deal of defects such as voids, etc., on account of which the electrical and optical properties of the film were seriously influenced. Moreover, little attention had been paid it as the material for studying basic physical properties, nor was there conducted research and development for its applications. However, at the beginning of the year 1976, a report was made for the first time as to possibility of forming a p-n junction in the a-Si, control of the p- and n-type conductivity of which had thereto fore been considered impossible (vide: Applied Physics Letter, Vol. 28, No. 2, pp 105-107, Jan. 15, 1976). Since then, attention has been drawn to this material and strenuous efforts have been exerted in research and development for its application to the solar battery, mainly. As a result of this, a patent has been granted in the United States (i.e., U.S. Pat. No. 4,064,521) for an invention concerning the photoconductive member for the solar battery.
However, since the a-Si film which has so far been reported and the a-Si film of a layer structure as taught in the above-described U.S. patent have been developed for use in the solar battery, they cannot be effectively used as the photoconductive layer for solid-state image pick-up devices, image original reading devices, electrophotographic image forming members, etc. in view of their electrical and optical characteristics.
In other words, according to various experiments done by the present inventors, it has so far been discovered that, while the a-Si film has a number of advantages as the photoconductive layer forming material for the electrophotographic image forming member in comparison with those conventional photoconductive materials such as inorganic photoconductive materials like Se, CdS, ZnO, or organic photoconductive materials (OPC) like poly-N-vinyl carbazole (PVCz) and trinitrofluorenone (TNF), there still are many problems to be solved before a single-layer electrophotographic image forming member formed of the a-Si material which has been developed for the purpose of the solar battery is available. The a-Si film is remarkably fast in its dark decay, even when this a-Si layer is subjected to the charging treatment for the electrostatic image formation. Accordingly the ordinary electrophotographic method is difficult to accomplish. This tendency towards dark decay is considerable in a highly humid atmosphere, and, depending on the case, the a-Si layer is utterly incapable of holding the electric charge.
Apart from the abovementioned electrophotographic image forming member using the a-Si, there has recently been proposed a new type of electrophotographic image forming member, the photoconductive layer of which is made of hydrogenated amorphous silicon (hereinafter abbreviated as "a-Si:H") as disclosed in, for example, DOLS No. 2746967 and DOLS No. 2855718.
The electrophotographic image forming member as disclosed in these prior applications has a number of excellent properties in comparison with the aforementioned electrophotographic image forming member. In other words, the photoconductive layer of either polarity, i.e., p-type or n-type, can be fabricated depending on the manufacturing conditions; the image forming member is prefectly free from liability to environmental pollution; it is excellent in its abrasion-resistant property due to its high surface hardness; it is also excellent in its developer-resistant property; and it is further excellent in its other electrophotographic properties such as cleaning property, moisture-resistant property, and so on.
Even with the a-Si:H type electrophotographic image forming member excellent in its electrophotographic characteristics in various points as mentioned above, there still exist rooms for improvement in respect of is photosensitivity in a practical light quantity region, its .gamma. value, its dark resistivity, its heat-resistant property in a much higher temperature region than a temperature region for ordinary use when conducting a process for improving the characteristics thereof or adding other functions thereto, and its photoresponse property, etc.
The present invention has been made in view of the afore-described various points of problem, and is based on the finding, as the result of continued strenuous efforts in researches and studies from very general standpoints of adaptability and applicability of the a-Si as the photoconductive member for use in the electrophotographic image forming member, the image pick-up device, image original reading device, etc., that, when two layers having mutually different electrical characteristics and comprising an amorphous material with silicon as a matrix and hydrogen as a constituent atom, i.e., hydrogenated amorphous silicon (a-Si:H), are laminated in a particular relationship, the photoconductive member to be obtained is not only useful practically, but also is superior to the conventional photoconductive materials in almost all aspects, in particular, its remarkably superior characteristics as the photoconductive member for electrophotography.